For many years, the advantages of providing a cargo carrying vehicle such as a truck with a lift gate has been recognized. As is well known, lift gates are gates which not only close the rear of the bed of the vehicle, but which, when in a horiztontal position, may be moved between a first position wherein they are an extension of the truck bed and a second position resting on the underlying terrain. Cargo may be loaded on or removed from the gate while in the latter position or loaded into or unloaded from the vehicle bed when in the former position.
Conventionally, some sort of motor is utilized to drive the lift gate between the two positions and there will generally be provided some sort of linkage which maintains the upper surface of the gate in a generally horizontal plane during such movement to prevent cargo on the gate from falling off.
Early versions of lift gates were generally not suited for small or medium sized cargo carrying vehicles because the linkage and the drive system for moving the gate between upper and lowered positions was adapted to be located wholly below the plane of the truck bed. This, in turn, required that the bed of the truck be relatively high above the underlying terrain to provide the requisite ground clearance; and this in turn essentially limited the provision of lift gates to relatively large cargo carrying vehicles with high road clearances.
To overcome this difficulty, in the 1960's, William D. Brown, in his U.S. Pat. No. 3,305,112, proposed a lift gate assembly that could be fitted to relatively small vehicles such as pick-up trucks, vans or the like. Brown proposed the use of a U-shaped base having upstanding legs. The top of the base was in the plane of the truck bed and immediately below it was a horizontally extending hydraulic cylinder acting on a cable system for raising and lowering the gate. The linkage for controlling gate position during raising and lowering was generally mounted at or above the plane of the truck bed within the upstanding legs of the base assembly and connected to the gate above the plane thereof via upstanding legs to which the gate was secured.
Brown's lift gate assembly worked very well for its intended purpose and was commercially successful. Nonetheless, it had a number of drawbacks.
Because the Brown lift gate was intended as an accessory for addition to pick-up trucks, efforts were made to minimize its weight. The reduced weight factor coupled with the particular form of hydraulic elevating drive for the gate employed frequently made it difficult to lower the gate in cold conditions.
Secondly, and again because of weight considerations resulting in a gate with considerably less rigidity than that found in previous lift gate assemblies used with large trucks, there was the possibility for undesirable torsional twisting from one side of the gate to the other when a load was not centered thereon. This "tipiness" contributed to considerable instability.
Furthermore, the Brown gate elevating system tended to generate the greatest lifting power when the gate was near its uppermost position whereas, it would be desirable to have the greatest lifting power available when the gate was at its lowermost position whereat it must not only be able to lift the load, it must also be able to overcome inertia to initiate the lifting process.
The present invention is directed to overcoming one or more of the above problems.